MEMS device manufacturing includes, among other things, the construction of physical elements that are added to traditional solid-state circuitry on chips known as integrated circuits (“ICs”). These physical elements can add significantly to the functional capability of ICs. For example, accelerometers, gyroscopes, gas density sensors, chemical or enzyme sensors, optical projection or redirection devices, micro-pumps, and pressure sensors have been fabricated. Usually, the physical motion or reaction of the element to external forces is measured through changes in capacitance, inductance, intrinsic resistance, etc. In other instances, the MEMS element can cause physical action rather than merely measuring physical forces. For example, such as in micro-pumping or mirror angle change.
At first the construction methodology to form these physical elements were reactive ion etching (RIE) and chemical bath etching processes in chemicals such as hydrofluoric acid (“HF”), mainly because the technology needed to carry out these methodologies were readily available from IC fabs. The features that could be produced with these methods and equipment tended to be purely vertical. If the features became largely horizontal and/or undercut, the features had to be large enough so liquid could be withdrawn from the undercut spaces in the substrate at the end of the liquid bath processing. As undercut features became smaller and smaller, withdrawing or drying out the small capillaries or undercut features became more challenging. Liquid to wall adhesion, or surface tension, tends to cause collapse of the thin overhanging features, thereby destroying the device. This defect has been named “stiction.”
In addition to stiction, other challenges and disadvantages are associated with HF bath etching, namely the inability to control material etching selectivity (oxide1 vs. oxide2). Still another challenge relates to selectivity of the metals that can be used to make contacts to the MEMS elements such as Aluminum, Copper, Gold, Silver, Titanium Nitride and other conductive materials.
In an attempt to help eliminate some of the disadvantages noted above in early MEMS etching/construction techniques, an HF and alcohol etching process was developed in the early 1990s as an alternative to the traditional HF water etch process. The HF/alcohol etching process helped solve some of the purely chemical issues but did not help with the “stiction” problem, especially as features started to shrink toward the 10 or even 1 micron scale. While solid state circuit elements were already being manufactured in the micron scale at that time, MEMS are physical elements which presented special problems that inhibited production at such a scale. As such, a better process was needed that could etch small pathways without creating “stiction” and without attacking adjacent materials.
In the middle 1990s, a process called Super-Critical-CO2 drying was developed. This process, which operates at about 2000 PSI and at about 30 degrees C. and uses carbon dioxide (“CO2”) gas, is able to remove liquids from small capillaries. This process was marginally successful but was cumbersome because it required a separate piece of equipment that allowed the MEMS chips to partially dry during transport to the supercritical CO2 equipment, thereby causing “stiction.” Therefore, problems remained.
A gas phase, atmospheric pressure HF/alcohol process was then developed to eliminate some of the problems with wet etching. Although the gas phase HF/alcohol was able to etch in small places, it often left liquid residues based on the byproducts of the etch since the byproduct vapor pressure was so low compared to the processing pressure. In order to remedy this problem, a gas phase HF/alcohol at reduced pressure was developed. An example of a reduced pressure gas phase HF/alcohol etching process for MEMs substrates is set forth in U.S. Pat. No. 5,439,553, Grant et al., the entirety of which is incorporated by reference.
The reduced pressure gas phase HF/alcohol processes overcame many of the deficiencies of the atmospheric HF/Alcohol process, such as the elimination of “stiction” and the minimization of attack of companion materials such as metals. The reduced pressure process was developed in single-substrate reactors/process chambers, which in turn were integrated into cluster tools using, for example, a Brooks Automation robotic handler. A typical example of a single-substrate processing reactor in which the reduced pressure gas phase HF/alcohol etching is performed is disclosed in U.S. Pat. No. 5,228,206, Grant et al., the entirety of which is incorporated by reference. In order to control “stiction” and selectivity, however, the application of the HF/Alcohol gas at reduced pressure proved to be a fairly lengthy process, taking 20 to 30 minutes or longer to achieve complete release etching. In order to become viable in production it was thought that the process needed to be reduced to 4 minutes. Moreover, etch uniformity was critical and had to be maintained well ahead of the 10% uniformity limit throughout the substrate. This proved to be difficult due to the construction requirements of cluster tool applications and the non-uniformity in processing conditions within existing MEMs processing tools. The non-uniformity of process conditions were in part due to the substrate loading and unloading requirements and other process control design criteria, which placed a burden on fluid dynamics and temperature differentials.
Batch processing of substrates is one way in which the throughput of substrate processing in the field of ICs has been increased. However, batch processing of substrates in the IC field involves wet processing techniques, which, for the reasons set forth above, can not be used for constructing MEMS. Additionally, the requirements for etching uniformity in MEMS construction is much stricter than that in IC manufacturing. Thus, reactor systems that can process batches of substrates for MEMS construction with acceptable production yield do not exist.
Despite these hurdles, the present inventor undertook the task of designing and building a reactor/process chamber system for the batch processing of substrates for MEMS construction, and specifically for implementing reduced pressure gas phase HF/alcohol etching processes, which depend heavily on the vapor pressures and partial pressures of the constituent gases. The problem in implementation however came in the form of substrate-to-substrate processing uniformity. More specifically, initial reactor system designs and processing conditions resulted in the top and bottom substrates in a stack of substrates being etched differently than the rest of the batch. Also, typical “gate opening” features where substrates are loaded into the process chamber of the reactor created a swirl or disturbance of the gas flow in the reactor. FIG. 1 illustrates a MEMS reactor 10 having a gate opening 11 in the process chamber that causes a swirling action of the process gases about the substrate 12, during processing. This swirling has proven to cause a recirculation pattern which affects the fluid boundary layer and the etch uniformity on the substrate. Also, control of the temperature of purely stainless steel or nickel reactors (required due to material compatibility with HF gas) was difficult. The variability of the processing had to be overcome in order to etch MEMS features uniformly across all substrates.